(FILES) In this file photo taken on September 22, 2020 A rocket launches from a S-400 missile system at the Ashuluk military base in Southern Russia during the "Caucasus-2020" military drills gathering China, Iran, Pakistan and Myanmar troops, along with ex-Soviet Armenia, Azerbaijan and Belarus. The United States on December 14, 2020 imposed sanctions on Turkey's military procurement agency after the NATO ally defiantly bought Russia's S-400 air defense system. / AFP / Dimitar DILKOFF
A rocket launches from a S-400 missile system at the Ashuluk military base in Southern Russia during the "Caucasus-2020" military drills gathering China, Iran, Pakistan and Myanmar troops, along with Show more

Explained: Turkey's controversial S-400 missile system purchase